The direct effects of oxytocin and vasopressin upon the vasculature of the liver and small intestine will be studied in dogs. Blood flows will be measured with non-cannulating electromagnetic transducers and changes in resistance in the small intestinal, hepatic arterial and intrahepatic portal venous vessels followed. Systemic arterial pressure, heart rate and intestinal motility will also be recorded. The phenomenon of tachyphylaxis to repeated administration of posterior pituitary hormones will be investigated. The interaction of these hormones with autonomic mechanisms will be explored using several different experimental protocols. The combined action of pituitary and ovarian hormones at peripheral vasomotor receptor sites will also be studied.